At the present time, light-polarizing films are usually produced by the absorption of polarizing coloring matter, such as iodine or dichroism dye, into a stretched and oriented film made of polyvinyl alcohol or its derivative or into an oriented polyene film prepared by dehydrochlorinating a polyvinyl chloride film or dehydrating a polyvinyl alcohol film to form a polyene film and then orienting the polyene film.
As for the performances of light-polarizing film, light-polarizing films having a high light-transmittance of single polarizer and a high polarizing co-efficient and showing a uniform light transmittance of single polarizer and a uniform polarizing co-efficient over the whole wavelength range of visible light, i.e. 400 nm to 700 nm, are desirable.
Among the light-polarizing films, those in which iodine is used as polarizing coloring matter show a uniform light transmittance (this means that the film assumes a neutral gray) over the whole wavelength range of visible light, i.e. 400 nm to 700 nm, and show a relatively uniform and high polarizing co-efficient, when the degree of dyeing is high, i.e. when light transmittance of single polarizer is low. However, when the degree of dyeing is low, i.e. when the light transmittance of single polarizer is high, the light transmittance cannot be uniform (in other words, neutral gray cannot be assumed) over the whole wavelength range of visible light (400 nm to 700 nm) and the polarizing co-efficient is low and dependent on wavelength. In this case, the absorption characteristics and polarization characteristics are particularly bad in the low wavelength region (400-500 nm) and high wavelength region (600-700 nm). Thus, in this case, light transmittance of single polarizer is higher and polarizing co-efficient is lower in said low and high wavelength regions than in other wavelength regions.
On the other hand, light-polarizing films in which a dichroism dye is used as polarizing coloring matter can show no constant light transmittance over the whole wavelength range of visible light (400-700 nm) and their polarizing co-efficient is irregular and dependent on wavelength, regardless of the degree of dyeing.